Galvanized steel sheet and method for manufacturing the same

ABSTRACT

A method for manufacturing a galvanized steel sheet, includes: galvanizing a steel sheet; bringing the surface of the steel sheet into contact with an aqueous solution containing zinc ion in the range of 5 to 100 g/l as the zinc ion concentration, having a pH of 4 to 6, and having a liquid temperature of 20 to 70° C., holding the steel sheet for 1 to 60 seconds; and then washing and drying the steel sheet. The solution containing zinc is preferably one containing zinc sulfate, for example. According to the method, a galvanized steel sheet having an oxide layer having an average thickness of 10 nm or more and mainly containing zinc formed on the surface of the steel sheet and having excellent press formability can be stably manufactured in a short time.

TECHNICAL FIELD

The present invention relates to a method for stably manufacturing agalvanized steel sheet having a low sliding resistance during pressforming and excellent press formability and a galvanized steel sheethaving excellent press formability.

BACKGROUND ART

The galvanized steel sheet has been widely utilized in wide rangingfields focusing on the application to automobile bodies. A galvanizedsteel sheet in such application is press formed for use. However, thegalvanized steel sheet has a disadvantage in that the press formabilityis inferior to that of a cold-rolled steel sheet. This is because thesliding resistance of the galvanized steel sheet in a press die ishigher than that of the cold-rolled steel sheet. More specifically, thegalvanized steel sheet becomes difficult to flow into a press die in aportion where the sliding resistance between a die and a bead, whicheasily causes fracture of the steel sheet.

Here, particularly a galvannealed steel sheet that has been subjected toalloying treatment after hot dip galvanizing treatment among galvanizedsteel sheets has more excellent weldability and coatability than thoseof a hot-dip zinc-plated steel sheet that has not been subjected toalloying treatment, and thus has been more preferably used as automobilebodies.

The galvannealed steel sheet is one in which an Fe—Zn alloy phase isformed by galvanizing a steel sheet, and heating the same so that Fe inthe steel sheet and Zn in a plating layer are dispersed to cause analloying reaction. The Fe—Zn alloy phase is a coating film generallycontaining a Γ phase, a δ₁ phase, and a ξ phase and has a tendency thatthe hardness and the melting point decrease with a reduction in the Feconcentration, i.e., in the order of Γ phase→δ₁ phase→ξ phase.Therefore, a coating film having a high Fe concentration in which thehardness is high, the melting point is high, and adhesion is difficultoccur is effective from the viewpoint of slidability. A galvannealedsteel sheet in which the press formability is emphasized is manufacturedin such a manner that the average Fe concentration in the coating filmis slightly high.

However, the coating film having a high Fe concentration has problems inthat the Γ phase that is hard and brittle is easily formed on theplated-steel sheet interface and a phenomenon of separation from theinterface during processing, i.e., a so-called powdering, is likely tooccur.

As methods for solving the problems, Patent Document 1 and PatentDocument 2 disclose a technique of increasing the weldability and theprocessability by subjecting the surface of a galvanized steel sheet toelectrolysis treatment, immersion treatment, coating oxidationtreatment, or heat-treatment to form an oxide film mainly containingZnO.

However, when the techniques of Patent Document 1 and Patent Document 2are applied to a galvannealed steel sheet, the surface reactivitybecomes poor due to the presence of an Al oxide and an effect ofimproving the press formability cannot be stably obtained because thesurface irregularities are large. More specifically, since the surfacereactivity is low, it is difficult to form a given film on the surfaceeven when the electrolysis treatment, immersion treatment, coatingoxidation treatment, heat-treatment, or the like is performed and thefilm thickness is small in a portion where the reactivity is low, i.e.,a portion in which the number of Al oxides is large. Since the surfaceirregularities are large, the surface convex portions directly contact apress die during press forming. The sliding resistance in contactportions of thin portions of the convex portions and the die becomeslarge, and thus an effect of improving the press formability is notsufficiently obtained.

Patent Document 3 discloses a technique of forming an oxide layer on aplated surface layer by hot dip galvanizing a steel sheet, alloying thesame by heat treatment, subjecting the resultant steel sheet to temperrolling, bringing the same into contact with an acidic solution havingpH buffer action, holding the same for 1 to 30 seconds, and then washingwith water.

Similarly, as a method for uniformly forming an oxide layer on a surfaceflat portion of a hot dip galvanized steel sheet that has not beensubjected to alloying treatment, Patent Document 4 discloses a methodincluding bringing a hot dip galvanized steel sheet after temper rollinginto contact with an acidic solution having pH buffer action, holdingthe same for a given period of time in a state where a liquid film ofthe acidic solution is formed on the surface of the steel sheet, andthen washing with water and drying the same.

Patent Document 1: Japanese Unexamined Patent Application PublicationNo. 53-60332

Patent Document 2: Japanese Unexamined Patent Application PublicationNo. 2-190483

Patent Document 3: Japanese Unexamined Patent Application PublicationNo. 2003-306781

Patent Document 4: Japanese Unexamined Patent Application PublicationNo. 2004-3004

When the techniques disclosed in Patent Document 3 and Patent Document 4are applied, favorable press formability can be obtained under formermanufacturing conditions. However, in recent years, the development of amanufacturing method for generating a thicker oxide film in a shorterperiod of time has been demanded in order to increase the productivity.When performed under such conditions, a sufficient oxide film is notformed and favorable press formability is not obtained in some cases inthe techniques disclosed in Patent Document 3 and Patent Document 4.

In view of such circumstance, it is an object of the present inventionto provide a method capable of stably manufacturing a galvanized steelsheet having excellent press formability even in a short time and agalvanized steel sheet having excellent press formability.

DISCLOSURE OF INVENTION

The present inventors have repeatedly conducted extensive research inorder to solve the problems. As a result, the following findings havebeen obtained.

The acidic solution for use in the techniques of Patent Document 3 andPatent Document 4 has pH buffer action in order to promote thedissolution of zinc. Therefore, it is considered that an increase in thepH is delayed, and thus the formation of an oxide layer is delayed. Inorder to compensate zinc for forming an oxide layer with zinc elutingfrom a plated coating film, an elution time of zinc is included in ageneration time of the oxide film. As a result, it is considered thatgenerating a thick oxide film in a short time becomes difficult.

Then, the present inventors have devised a technique of generating anoxide film in a shorter time by omitting an elution time of zinc byblending zinc ion in an aqueous solution for generating an oxide filmbeforehand. However, the formation of an oxide film has not beenpromoted merely by blending zinc ion in an aqueous solution beforehand.Particularly in the case where the pH is 2 described in Examples ofPatent Document 3 and Patent Document 4, even when zinc is blended in atreatment liquid, the formation of an oxide film has not been promoted.

This is considered to be because, according to the techniques of PatentDocument 3 and Patent Document 4, an environment is established in whicha zinc oxide is likely to generate because the pH near the surfaceincreases due to the reduction of hydrogen ion occurring simultaneouslywith the elution of zinc, but the pH near the surface does not increasemerely by blending zinc ion in an aqueous solution, and thus anenvironment is not established in which a zinc oxide is likely togenerate.

Then, the present inventors have devised a technique of setting the pHof an aqueous solution to 4 to 6, the pH at which a zinc oxide is likelyto generate. Then, the present inventors have found that, by setting thepH of a treatment liquid to 4 to 6, zinc is generated as a hydroxide dueto a slight increase in the surface pH caused by slight elution of zincof a plated coating film.

The present invention has been accomplished based on the findings, andthe gist is as follows.

[1] A method for manufacturing a galvanized steel sheet, includesgalvanizing a steel sheet, bringing the steel sheet into contact with anaqueous solution, holding the steel sheet for 1 to 60 seconds after thetermination of the contact treatment, and then washing with water anddrying the steel sheet to thereby form an oxide layer on the surface ofthe steel sheet, in which the aqueous solution for use in the contacttreatment of the steel sheet contains zinc ion in the range of 5 to 100g/l as the zinc ion concentration, has a pH of 4 to 6, and has a liquidtemperature of 20 to 70° C.

[2] The method for manufacturing a galvanized steel sheet according to[1] above, in which the aqueous solution contains zinc sulfate.

[3] The method for manufacturing a galvanized steel sheet according to[1] or [2] above, in which a liquid film to be formed on the surface ofthe steel sheet after the steel sheet contacts the aqueous solution is 5to 30 g/m².

[4] A galvanized steel sheet, which is manufactured according to themethod for manufacturing a galvanized steel sheet according to any oneof [1] to [3] above, in which an oxide layer mainly containing zinc as ametal component is formed on the surface of the steel sheet in such amanner as to have an average thickness of 10 nm or more.

In the invention, the galvanized steel sheet is a plated steel sheethaving a coating film containing zinc as the main component formed onthe surface and includes a hot dip galvanized steel sheet (abbreviatedas a GI steel sheet), a galvannealed Steel Sheet (abbreviated as a GAsteel sheet), an electrogalvanized steel sheet (abbreviated as an EGsteel sheet), a vapor deposition galvanized steel sheet, an alloygalvanized steel sheet containing an alloy element of Fe, Al, Ni, MgCo,or the like, etc.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view of a principal part of an oxide layer formationtreatment facility used in Examples.

FIG. 2 is a schematic front view showing a friction coefficientmeasuring device.

FIG. 3 is a schematic perspective view showing the shape and the size ofa bead in FIG. 2.

FIG. 4 is a schematic perspective view showing the shape and the size ofthe bead in FIG. 2.

FIG. 5 is a view showing influence of the zinc ion concentration on theoxide film thickness.

BEST MODES FOR CARRYING OUT THE INVENTION

In the invention, when forming an oxide layer on the surface of a steelsheet by galvanizing a steel sheet, bringing the steel sheet intocontact with an aqueous solution, holding the steel sheet for 1 to 60seconds after the termination of the contact treatment, and then washingwith water and drying the steel sheet, the aqueous solution containszinc ion in the range of 5 to 100 g/l as the zinc ion concentration, thepH is 4 to 6, and the liquid temperature is 20 to 70° C. To prepare anaqueous solution containing zinc ion in a given concentration and havinga specified pH and a specified liquid temperature as described above asthe aqueous solution for use in the contact treatment of the steel sheetis an important requirement and a feature in the invention. Thus, anoxide layer sufficient for securing favorable press formability can beformed in a short time.

The “after the termination of the contact treatment” refers to “afterthe termination of an immersion process” in the case of immersiontreatment, “after the termination of a spraying process” in the case ofspraying treatment, and “after the termination of a coating process” inthe case of roll coating.

The use of an aqueous solution containing zinc ion as the aqueoussolution for use in the contact treatment of the steel sheet allowsomission of an elution time of zinc. In this case, the zinc ion is inthe range of 5 to 100 g/l as the zinc ion concentration. When the zincion concentration is lower than 5 g/l, sufficient zinc is not supplied,resulting in a failure of the formation of an oxide layer. In contrast,when the zinc ion concentration exceeds 100 g/l, the concentration ofsulfuric acid contained in the oxide layer to be formed becomes high,resulting in concern about contamination of a treatment liquid when theoxide dissolves in chemical conversion treatment to be carried outthereafter.

In order to form a stable zinc compound as an oxide layer, it ispreferable to add zinc ion as a sulfate. It is considered that when zincion is added as a sulfate, sulfuric acid ion is taken into an oxidelayer to be formed to thereby produce an effect of stabilizing the oxidelayer.

As described above, the formation of an oxide film is not promotedmerely by blending zinc ion in a treatment liquid beforehand. Then, inthe invention, the pH needs to be set to 4 to 6, at which a zinc oxideeasily generates. When the pH of a treatment liquid is set to 4 to 6,zinc generates as a hydroxide due to a slight increase in the surface pHcaused by slight elution of zinc of a plated coating film. As a resultthereof, the zinc elution time can be omitted and the generation of azinc oxide can be achieved. When the pH exceeds 6, zinc ion precipitatesin the aqueous solution (formation of a hydroxide) and is not formed asan oxide on the surface of the steel sheet. When the pH is lower than 4,the formation of the oxide layer is hindered due to the delay of anincrease in the pH as described above.

The temperature of the aqueous solution is 20 to 70° C. Since the oxidelayer formation reaction occurs when holding the steel sheet in a givenperiod of time after contacting the aqueous solution, it is effective tocontrol the sheet temperature during holding in the range of 20 to 70°C. When the sheet temperature is lower than 20° C., a long period oftime is required for the oxide layer generation reaction, resulting in areduction in the productivity. In contrast, when the sheet temperatureexceeds 70° C., a reaction relatively quickly proceeds but treatmentunevenness is likely to occur on the surface of the steel sheet.

The aqueous solution used in Patent Document 3 and Patent Document 4 hasa feature in that the aqueous solution is acidic and has pH bufferaction. In the invention, however, since an aqueous solution containingzinc ion is used, a sufficient oxide layer can be formed even when thedissolution of zinc is not caused by increasing the pH of the aqueoussolution. A prompt increase in the pH is considered to be advantageousfor the formation of an oxide. Therefore, the pH buffer action is notnecessarily indispensable.

In the invention, an oxide layer excellent in slidability can be stablyformed when zinc is contained in the aqueous solution contacting thesurface of the steel sheet. Therefore, even when other metal ions,inorganic compounds, and the like are contained as impurities orintentionally contained in the aqueous solution, the effects of theinvention are not impaired. Even when N, P, B, Cl, Na, Mn, Ca, Mg, Ba,Sr, Si, and the like are taken into the oxide layer, it can be appliedinsofar as the effects of the invention are not impaired.

Preferably, after bringing a galvanized steel sheet into contact withthe aqueous solution containing the above, the aqueous solution ispresent on the surface of the steel sheet in the form of a thin liquidfilm. This is because when the amount of the aqueous solution present onthe surface of the steel sheet is large, the pH of the aqueous solutionis hard to increase even when the dissolution of zinc occurs, and a longperiod of time is required for the formation of the oxide layer. Fromthis viewpoint, it is preferable and effective to adjust the amount ofan aqueous solution film to be formed on the surface of the steel sheetto 30 g/m² or lower. In order to prevent the liquid film from drying,the amount of the liquid film of 5 g/m² or more is suitable. Asdescribed above, the liquid film to be formed on the surface of thesteel sheet after contacting the aqueous solution is preferably 5 to 30g/m². The adjustment of the amount of the aqueous solution film can beperformed by a squeeze roll, air wiping, or the like.

The time (retention time before washing with water) before washing withwater after immersion in the aqueous solution is 1 to 60 seconds. Whenthe time before washing with water is lower than 1 second, the aqueoussolution is washed away before a sufficient oxide layer is formed, andthus an effect of improving the slidability is not obtained. Incontrast, when the time before washing with water exceeds 60 seconds,the productivity decreases. Since the object of the invention is tostably manufacture a galvanized steel sheet even in a short time, theretention time is 60 seconds or lower for sufficiently demonstrating theeffects of the invention.

As described above, on the surface of the plated steel sheet of theinvention, an oxide layer mainly containing zinc as a metal componentand having an average thickness of 10 nm or more is obtained.

The “mainly containing zinc” refers to containing zinc in a proportionof 50% by mass or more as a metal component.

The oxide layer in the invention refers to a layer containing an oxideand/or a hydroxide mainly containing zinc as a metal component. Theaverage thickness of the oxide layer is required to be 10 nm or more.When the average thickness of the oxide layer is small, e.g., lower than10 nm, an effect of reducing sliding resistance becomes insufficient. Incontrast, when the average thickness of the oxide layer containing zincas an essential ingredient exceeds 100 nm, there is a tendency that thecoating film breaks during press processing, the sliding resistanceincreases, and the weldability decreases. Thus, such a thickness is notpreferable.

Methods for bringing the galvanized steel sheet into contact with theaqueous solution containing zinc are not particularly limited. Forexample, a method for immersing the plated steel sheet in the aqueoussolution, a method for spraying the aqueous solution to the plated steelsheet, a method for applying the aqueous solution to the plated steelsheet with a coating roll, and the like are mentioned. It is preferablefor the aqueous solution to be finally present on the surface of thesteel sheet in the form of a thin liquid film.

For manufacturing the galvannealed steel sheet according to theinvention, Al needs to be added into a plating bath but additionalelement ingredients other than Al are not particularly limited. Morespecifically, even when Pb, Sb, Si, Sn, Mg, Mn, Ni, Ti, Li, Cu, and thelike other than Al are contained or added, the plating bath can beapplied insofar as the effects of the invention are not impaired.

EXAMPLES

Next, the invention will be described in more detail with reference toExamples.

A GI steel sheet was produced by performing hot dip galvanizing in whichthe deposit amount per surface was 45 g/m² and the Al concentration was0.20% by mass on a cold-rolled steel sheet having a sheet thickness of0.8 mm, and then performing temper rolling. A GA steel sheet wasobtained by forming a plated coating film in which the deposit amountper surface was 45 g/m², the Fe concentration was 10% by mass, and theAl concentration was 0.20% by mass on a cold-rolled steel sheet having asheet thickness of 0.8 mm by a standard galvannealing method, andfurther performing temper rolling. An EG steel sheet was produced byhaving a plated coating film having a deposit amount per surface of 30g/m² on a cold-rolled steel sheet having a sheet thickness of 0.8 mm bya standard electrogalvanizing method.

Subsequently, an oxide layer was formed using a treatment facilityhaving a structure shown in FIG. 1. First, steel sheets S, such as theGI steel sheet, the GA steel sheet, and the EG steel sheet obtainedabove were immersed in aqueous solutions in which the treatment liquidcomposition, the temperature, and the pH were different from each otheras shown in Tables 1-1 and 1-2 in a solution bath 2. Subsequently, theamount of liquid films on the surface of the steel sheets was adjustedwith a squeeze roll 3. The adjustment of the amount of liquid films wasperformed by changing the pressure of the squeeze roll. Subsequently,the steel sheets were made to pass through a washing bath 5 and awashing bath 6 without being treated, hot water of 50° C. was sprayed tothe steel sheets in a washing bath 7 for washing, and the steel sheetswere dried with a drier 8, so that an oxide layer is formed on theplated surface. A washing bath 1 can be provided before the solutionbath 2.

As the aqueous solution for use in the immersion treatment in thesolution bath 2, an aqueous solution was used to which a given amount ofzinc sulfate heptahydrate was added in order to add zinc ion. Forcomparison, a solution containing 20 g/L of sodium acetate whose pH wasadjusted with sulfuric acid was also used in some cases.

The retention time before washing with water was the time before washingin the washing bath 7 was started after adjusting the amount of liquidfilms with the squeeze roll 3 and was adjusted by changing the linespeed. Some of the steel sheets were produced by washing immediatelyafter squeezing using a shower washing device 4 at the exit side of thesqueeze roll 3.

Next, the steel sheets produced as described above were judged whetheror not they have an appearance sufficient as an exterior panel forautomobiles, and also the measurement of a friction coefficient as amethod for simply evaluating the press formability and a spherical headbulging test was carried out in order to simulate the actual formabilityin detail were carried out. The measurement methods are as follows.

(1) Press Formability Evaluation Test (Friction Coefficient MeasurementTest)

In order to evaluate the press formability, the friction coefficient ofeach test piece was measured as follows.

FIG. 2 is a schematic front view showing a friction coefficientmeasuring device. As shown in FIG. 2, a friction coefficient measuringsample 11 extracted from the test piece is fixed to a sample stand 12.The sample stand 12 is fixed to the upper surface of a horizontallymovable slide table 13. On the lower surface of the slide table 13, avertically movable slide table support stand 15 having a roller 14contacting the lower surface of the slide table 13. By pressing up thesame, a first load cell 17 for measuring a pressing load N to thefriction coefficient measuring sample 11 by a bead 16 is attached to theslide table support stand 15. In order to measure a sliding resistance Ffor horizontally moving the slide table 13 along a rail 19 in the statewhere the pressing force was made to act, a second load cell 18 isattached to one end of the slide table 13. As a lubricant, a cleaningoil for pressing, Preton R352L manufactured by Sugimura ChemicalIndustrial Co., Ltd., was applied onto the surface of the frictioncoefficient measuring sample 11, and thus a test was carried out.

FIGS. 3 and 4 are schematic perspective view showing the shape and thesize of the used bead. The bead 16 slides while the lower surface of thebead 16 being pressed against the surface of the sample 11. In the bead16 shown in FIG. 3, the width is 10 mm, the length in the slidingdirection of the sample is 12 mm, and each end in the sliding directionof the lower surface of the bead 16 is curved with a curvature of 4.5mmR. The lower surface of the bead 16 against which the sample ispressed has a plane with a width of 10 mm and a length in the slidingdirection of 3 mm. In the bead 16 shown in FIG. 4, the width is 10 mm,the length in the sliding direction of the sample is 69 mm, and each endin the sliding direction of the lower surface of the bead 16 is curvedwith a curvature of 4.5 mmR. The lower surface of the bead 16 againstwhich the sample is pressed has a plane with a width of 10 mm and alength in the sliding direction of 60 mm.

The friction coefficient measurement test was carried out under twoconditions shown below.

[Condition 1]

The bead shown in FIG. 3 was used, the pressing load N was 400 kgf, andthe sample drawing rate (horizontal movement rate of the slide table 13)was 100 cm/min.

[Condition 2]

The bead shown in FIG. 4 was used, the pressing load N was 400 kgf, andthe sample drawing rate (horizontal movement rate of the slide table 13)was 20 cm/min.

The friction coefficient between the test piece and the bead wascalculated based on Equation: μ=F/N.

(2) Spherical Head Bulging Test

A test piece having a size of 200×200 mm was subjected to bulge formingusing a 150 mmφ punch by a liquid pressure bulge testing machine. Then,the maximum forming height when the test piece was broken was measured.During the test, a wrinkle pressing force of 100 Ton was applied inorder to prevent inflow of materials, and a lubricating oil was appliedonly to the surface which the punch contacted. The used lubricating oilis the same as that of the friction coefficient measurement testdescribed above.

(3) Measurement of Thickness of Oxide Layer (Oxide Film Thickness)

An Si wafer on which a thermal oxidation SiO₂ film having a filmthickness of 96 nm was formed was used as a reference substance, and theaverage thickness of the oxide layer in terms of SiO₂ was determined bymeasuring the O·Kα·X rays by an x-ray fluorescence spectrometer. Theanalysis area is 30 mmφ.

The test results obtained in the above are shown in Tables 1-1 and 1-2.

TABLE 1-1 Time Oxide Friction Used solution Solu- Liquid before filmcoefficient Maxium Steel Zn tion film water thick- Con- Con- formingsheet Test pH concen- Temper- amount washing ness dition dition heightappear- No. piece buffer tratio pH ature (g/m²) (second) (nm) 1 2 (mm)ance Remarks  1 GA — — — — — —  8 0.180 0.223 35.0 ∘ Comparative example1  2 Sodium — 2.0 35° C. 10 10 15 0.149 0.190 36.5 ∘ Comparative example2  3 acetate sulfuric 10 30 30 0.128 0.165 38.1 ∘ Comparative example 3 4 (20 g/L) acid 10 60 42 0.120 0.163 39.3 ∘ Comparative example 4 added 5 — 5.0 35° C. 10 10  8 0.183 0.219 35.6 ∘ Comparative example 5  6sulfuric 10 30  8 0.179 0.221 35.9 ∘ Comparative example 6  7 acid 10 60 8 0.180 0.217 35.9 ∘ Comparative example 7 added  8 — 2.5 g/l 5.6 35°C. 10 10 12 0.148 0.200 36.5 ∘ Comparative example 8  9 10 30 25 0.1400.174 37.9 ∘ Comparative example 9 10 10 60 32 0.132 0.163 38.9 ∘Comparative example 10 11   5 g/l 5.5 35° C. 10 10 18 0.138 0.187 37.6 ∘Example of present invention 1 12 10 30 32 0.123 0.166 39.1 ∘ Example ofpresent invention 2 13 10 60 43 0.122 0.163 39.5 ∘ Example of presentinvention 3 14  10 g/l 5.2 35° C. 10 10 25 0.134 0.174 39.0 ∘ Example ofpresent invention 4 15 10 30 35 0.128 0.164 39.4 ∘ Example of presentinvention 5 16 10 60 45 0.124 0.163 40.1 ∘ Example of present invention6 17  50 g/l 5.0 35° C. 10  0 12 0.160 0.205 37.2 ∘ Comparative example11 18 10  1 15 0.145 0.200 38.1 ∘ Example of present invention 7 19 10 5 24 0.137 0.173 39.0 ∘ Example of present invention 8 20 10 10 320.127 0.165 38.9 ∘ Example of present invention 9 21 10 30 40 0.1270.159 39.9 ∘ Example of present invention 10 22 10 60 50 0.125 0.16040.8 ∘ Example of present invention 11 23 15° C. 10 10 14 0.145 0.19837.1 ∘ Comparative example 12 24 10 30 27 0.134 0.169 38.1 ∘ Comparativeexample 13 25 10 60 37 0.125 0.166 39.2 ∘ Comparative example 14 26 25°C. 10 10 19 0.138 0.189 38.5 ∘ Example of present invention 12 27 10 3032 0.129 0.166 40.3 ∘ Example of present invention 13 28 10 60 43 0.1270.162 40.0 ∘ Example of present invention 14 29 65° C. 10 10 35 0.1280.164 40.5 ∘ Example of present invention 15 30 10 30 44 0.125 0.16241.6 ∘ Example of present invention 16 31 10 60 51 0.123 0.160 41.8 ∘Example of present invention 17 32 75° C. 10 10 35 0.124 0.161 40.6 xComparative example 15 33 10 30 45 0.120 0.163 40.5 x Comparativeexample 16 34 10 60 52 0.121 0.158 41.0 x Comparative example 17 35 35°C. 30 10 20 0.143 0.185 38.5 ∘ Example of present invention 18 36 30 3034 0.128 0.163 39.1 ∘ Example of present invention 19 37 30 60 49 0.1270.161 40.4 ∘ Example of present invention 20 38 35° C. 40 10 13 0.1550.202 37.4 ∘ Example of present invention 21 39 40 30 28 0.132 0.16739.0 ∘ Example of present invention 22 40 40 60 42 0.127 0.163 39.7 ∘Example of present invention 23 41 3.5 35° C. 10 10 14 0.149 0.201 36.0∘ Comparative example 18 42 sulfuric 10 30 27 0.130 0.165 39.2 ∘Comparative example 19 43 acid 10 60 40 0.124 0.162 41.0 ∘ Comparativeexample 20 added 44 100 g/l  4.9 35° C. 10 10 34 0.125 0.166 40.5 ∘Example of present invention 24 45 10 30 41 0.123 0.164 40.9 ∘ Exampleof present invention 25 46 10 60 50 0.121 0.163 40.8 ∘ Example ofpresent invention 26

TABLE 1-2 Time Oxide Friction Used solution Solu- Liquid before filmcoefficient Maxium Steel Zn tion film water thick- Con- Con- formingsheet Test pH concen- Temper- amount washing ness dition dition heightappear- No. piece buffer tration pH ature (g/m²) (second) (nm) 1 2 (mm)ance Remarks 47 GI — — — — — —  7 0.175 0.215 35.7 ∘ Comparative example21 48 Sodium — 2.0 35° C. 10 10 13 0.147 0.187 36.7 ∘ Comparativeexample 22 49 acetate sulfuric 10 30 27 0.125 0.164 38.6 ∘ Comparativeexample 23 50 (20 g/L) acid 10 60 39 0.119 0.160 39.6 ∘ Comparativeexample 24 added 51 — 2.5 g/l 5.6 35° C. 10 10 11 0.161 0.198 36.3 ∘Comparative example 25 52 10 30 24 0.13  0.168 38   ∘ Comparativeexample 26 53 10 60 34 0.121 0.163 39.1 ∘ Comparative example 27 54  10g/l 5.2 35° C. 10 10 19 0.138 0.177 37.1 ∘ Example of present invention27 55 10 30 32 0.124 0.161 38.5 ∘ Example of present invention 28 56 1060 42 0.121 0.161 39.5 ∘ Example of present invention 29 57  50 g/l 5.035° C. 10 10 26 0.127 0.166 38.4 ∘ Example of present invention 30 58 1030 36 0.122 0.163 39   ∘ Example of present invention 31 59 10 60 450.12  0.159 39.7 ∘ Example of present invention 32 60 EG — — — — — —  90.146 0.289 33.7 ∘ Comparative example 28 61 Sodium — 2.0 35° C. 10 1011 0.136 0.241 34.0 ∘ Comparative example 29 62 acetate sulfuric 10 3023 0.135 0.195 36.6 ∘ Comparative example 30 63 (20 g/L) acid 10 60 360.128 0.173 37.9 ∘ Comparative example 31 added 64 — 2.5 g/l 5.6 35° C.10 10 12 0.140 0.199 37.0 ∘ Comparative example 32 65 10 30 23 0.1310.181 37.2 ∘ Comparative example 33 66 10 60 31 0.131 0.159 37.5 ∘Comparative example 34 67  10 g/l 5.2 35° C. 10 10 20 0.139 0.221 35.4 ∘Example of present invention 33 68 10 30 35 0.132 0.195 36.9 ∘ Exampleof present invention 34 69 10 60 45 0.129 0.160 37.6 ∘ Example ofpresent invention 35 70  50 g/l 5.0 35° C. 10 10 22 0.136 0.192 37.6 ∘Example of present invention 36 71 10 30 37 0.131 0.153 39.3 ∘ Exampleof present invention 37 72 10 60 46 0.125 0.156 38.9 ∘ Example ofpresent invention 38The following items were clarified from the test results shown in Tables1-1 and 1-2.

(1) Since Nos. 1, 47, and 60 were not treated with a solution, an oxidefilm sufficient for increasing the slidability was not formed on theflat portion. Thus, the friction coefficient is high.

(2) Nos. 2 to 4, Nos. 48 to 50, and Nos. 61 to 63 are comparativeexamples using an acidic solution having pH buffer action. In the caseof the treatment of 30 seconds or more, the friction coefficient is lowand the maximum forming height is large but in the case of the treatmentof 10 seconds, a sufficient reduction in the friction coefficient and anincrease in the maximum forming height are not satisfied.

(3) Nos. 5 to 7 are comparative examples using an acidic solution havingpH buffer action. High friction coefficients are exhibited.

(4) Nos. 8 to 10, Nos. 51 to 53, and Nos. 64 to 66 are comparativeexamples in which zinc ion is contained but the amount is smaller thanthe range of the invention. In the case of the treatment of 30 secondsor more, the friction coefficient is low and the maximum forming heightis large but in the case of the treatment of 10 seconds, a sufficientreduction in the friction coefficient and an increase in the maximumforming height are not satisfied.

(5) Nos. 11 to 13, Nos. 54 to 56, and Nos. 67 to 69 are examples of theinvention that were treated with a solution containing zinc ion, inwhich the friction coefficient decreases and also the maximum formingheight increases. Nos. 14 to 16 and Nos. 44 to 46 are examples of theinvention in which the treatment conditions were the same as those ofNos. 11 to 13 and the zinc ion concentration in the liquid wasincreased. The friction coefficient becomes stable at lower levels andalso the maximum forming height further increases. Similarly, Nos. 57 to59 and Nos. 70 to 72 are examples of the invention in which thetreatment conditions were the same as those of Nos. 54 to 56 and thezinc ion concentration in the liquid was increased. The frictioncoefficient becomes stable at lower levels and also the maximum formingheight further increases.

(6) Nos. 17 to 22 are examples in which a solution film was formed onthe surface of the steel sheets and the time until washing with waterwas carried out was changed. In No. 17 which was washed with waterwithout being held, the friction coefficient merely slightly decreases.In contrast, in Nos. 18 to 22 in which the retention time was 1 secondor more, the friction coefficient decreases and also the bulgingproperties stably increase.

(7) Nos. 23 to 40 are examples in which the treatment liquid temperaturewas changed. In Nos. 23 to 25 having a low treatment liquid temperature,effects of improving the friction coefficient and the maximum formingheight are not sufficient as compared with the other examples. Incontrast, Nos. 32 to 34 are examples having a high treatment liquidtemperature and effects of improving the friction coefficient or themaximum forming height were sufficient but treatment unevenness wasobserved in many portions and thus the appearance was not favorable asan exterior panel for automobiles.

(8) Nos. 35 to 40 are examples of the invention in which the liquid filmformation amount was changed relative to Nos. 20 to 22. A comparisonbetween the samples in which the retention time until washing with waterwas carried out is the same shows that when the liquid film amount waslarge, a sufficient reduction in the friction coefficient and anincrease in the maximum forming height are achieved but the frictioncoefficient was slightly high and also the maximum forming height wassmall as compared with the samples in which the liquid film amount wassmall.

(9) Nos. 41 to 43 are comparative examples using a treatment liquid inwhich pH is lower than the range of the invention. The effect ofreducing the friction coefficient is not observed and also an increasein the maximum forming height is not observed as compared with Nos. 20to 22.

FIG. 5 is a view showing the influence of the zinc ion concentration onthe oxide film thickness using Nos. 8 to 22 and Nos. 44 to 46 of Tables1-1 and 1-2. FIG. 5 shows that the oxide film has a sufficient thicknesseven when the retention time is short (e.g., 10 seconds) in the casewhere the zinc concentration is 5 g/l or more, and the problem of theinvention that the oxide film thickness becomes small when the retentiontime is short is solved.

INDUSTRIAL APPLICABILITY

According to the present invention, a galvanized steel sheet having alow sliding resistance during press forming and excellent pressformability can be stably manufactured at a saved space even undershort-time manufacturing conditions. For example, even when a highstrength galvanized steel sheet which has a high forming load and islikely to cause die galling, the sliding resistance during press formingis low and excellent press formability can be achieved. Since the pressformability is excellent, the invention can be applied to wide rangingfields focusing on the application to automobile bodies.

1. A method for manufacturing a galvanized steel sheet, comprising:galvanizing a steel sheet; bringing the steel sheet into contact with anaqueous solution; holding the steel sheet for 1 to 60 seconds after thetermination of the contact treatment; and washing with water and dryingthe steel sheet to thereby form an oxide layer on the surface of thesteel sheet, the aqueous solution for use in the contact treatment ofthe steel sheet containing zinc ion in the range of 5 to 100 g/l as thezinc ion concentration, having a pH of 4 to 6, and having a liquidtemperature of 20 to 70° C.
 2. The method for manufacturing a galvanizedsteel sheet according to claim 1, wherein the aqueous solution containszinc sulfate.
 3. The method for manufacturing a galvanized steel sheetaccording to claim 1, wherein a liquid film to be formed on the surfaceof the steel sheet after the steel sheet contacts the aqueous solutionis 5 to 30 g/m².
 4. A galvanized steel sheet, which is manufacturedaccording to the method for manufacturing a galvanized steel sheetaccording to claim 1, an oxide layer mainly containing zinc as a metalcomponent being formed on the surface of the steel sheet in such amanner as to have an average thickness of 10 nm or more.
 5. The methodfor manufacturing a galvanized steel sheet according to claim 2, whereina liquid film to be formed on the surface of the steel sheet after thesteel sheet contacts the aqueous solution is 5 to 30 g/m².
 6. Agalvanized steel sheet, which is manufactured according to the methodfor manufacturing a galvanized steel sheet according to claim 2, anoxide layer mainly containing zinc as a metal component being formed onthe surface of the steel sheet in such a manner as to have an averagethickness of 10 nm or more.
 7. A galvanized steel sheet, which ismanufactured according to the method for manufacturing a galvanizedsteel sheet according to claim 3, an oxide layer mainly containing zincas a metal component being formed on the surface of the steel sheet insuch a manner as to have an average thickness of 10 nm or more.